Gastrointestinal dysfunction in mice with a targeted mutation in the gene encoding vasoactive intestinal polypeptide: a model for the study of intestinal ileus and Hirschsprung's disease

In 1970, Drs. Said and Mutt isolated a novel peptide from porcine intestinal extracts with powerful vasoactive properties, and named it vasoactive intestinal peptide (VIP). Since then, the biological actions of VIP in the gut as well as its signal transduction pathways have been extensively studied. A variety of in vitro and in vivo studies have indicated that VIP, expressed in intrinsic non-adrenergic non-cholinergic (NANC) neurons, is a potent regulator of gastrointestinal (GI) motility, water absorption and ion flux, mucus secretion and immune homeostasis. These VIP actions are believed to be mediated mainly by interactions with highly expressed VPAC(1) receptors and the production of nitric oxide (NO). Furthermore, VIP has been implicated in numerous physiopathological conditions affecting the human gut, including pancreatic endocrine tumors secreting VIP (VIPomas), insulin-dependent diabetes, Hirschsprung's disease, and inflammatory bowel syndromes such as Crohn's disease and ulcerative colitis. To further understand the physiological roles of VIP on the GI tract, we have begun to analyze the anatomical and physiological phenotype of C57BL/6 mice lacking the VIP gene. Herein, we demonstrate that the overall intestinal morphology and light microscopic structure is significantly altered in VIP(-/-) mice. Macroscopically there is an overall increase in weight, and decrease in length of the bowel compared to wild type (WT) controls. Microscopically, the phenotype was characterized by thickening of smooth muscle layers, increased villi length, and higher abundance of goblet cells. Alcian blue staining indicated that the latter cells were deficient in mucus secretion in VIP(-/-) mice. The differences became more pronounced from the duodenum to the distal jejunum or ileum of the small bowel but, became much less apparent or absent in the colon with the exception of mucus secretion defects. Further examination of the small intestine revealed larger axonal trunks and unusual unstained patches in myenteric plexus. Physiologically, the VIP(-/-) mice showed an impairment in intestinal transit. Moreover, unlike WT C57BL/6 mice, a significant percentage of VIP(-/-) mice died in the first postnatal year with overt stenosis of the gut.     

Fluid secretory responses to enterotoxin STa and 8-bromo-cyclic GMP in fed and nutrionally-deprived gerbils: jejunum, ileum and colon in vivo

Fluid transport was measured gravimetrically in vivo in the jejunum, ileum and colon of fed, fasting (four days) and undernourished (50 % of control food intake for 21 days) gerbils (Gerbillus cheesmani). The effects of luminal enterotoxin Escherichia coli STa (50 ng/ml) and luminal 8-bromo-cyclic GMP (cGMP 1 mM) on fluid transport across jejunum, ileum and colon were also assessed. Fasting and undernourishment reversed the normal basal fluid absorption measured in fed ileum and colon into secretion. Neither fasting nor undernourishment had any effect on jejunal basal fluid absorption. In jejuna, ilea and colons of fed animals as well as in jejuna from fasting and undernourished gerbils STa (50 ng) reversed the normal absorptive "tone" to secretion but it had no significant effects on fluid secretion in either the ileum or colon from fasted gerbils. STa increased significantly the fluid secretion in ileum from undernourished gerbils. Luminal cGMP had no effect on basal absorptive tone in the jejunum of fed and fasted gerbils, but reversed absorption into secretion in the jejuna from undernourished gerbils. In the ilea taken from fed animals the small basal absorption was reversed to secretion by luminal cGMP. Although cGMP produced no significant changes in fluid secretion in the ilea taken from fasted gerbils, yet it caused a significant increase in those from undernourished gerbils. In the colon taken from fed animals cGMP decreased the basal fluid absorption significantly, but it had no significant effect on fluid secretion in the colon of fasted or undernourished gerbils. We conclude that fasting and undernourishment have no significant effects on fluid transport across the gerbil jejunum but reversed basal absorption in the fed ileum and colon into secretion. cGMP mimic the effects of STa in the jejunum taken from undernourished gerbils, in the ileum obtained under the three nutritional states and in the colon taken from fasting animals.     

Antisecretory effects of somatostatin and vasopressin in the rat colon descendens in vitro

The effects of two hormones, vasopressin and somatostatin (SOM), on ion secretion in rat colon descendens were compared. Three modes for induction of epithelial secretion were used: neuronally mediated secretion due to electric field stimulation (EFS), Ca2+-dependent secretion elicited by carbachol, and cAMP-dependent secretion evoked either by a receptor-mediated mechanism elicited by vasoactive intestinal peptide (VIP) or by a direct activation of the adenylate cyclase by means of forskolin. Somatostatin inhibited ion secretion evoked by EFS (55-65%), carbachol (80%) and VIP (95%) in a dose-dependent manner. Maximal inhibition by SOM was observed at 10(-7) M. Somatostatin had, however, no effect on the secretory response to forskolin. The inhibition of the VIP effect could be attenuated by pretreatment with pertussis toxin. In contrast, vasopressin in concentrations as low as 0.025-0.25 U/liter decreased the secretory effects of EFS (55-75%) and carbachol (85%), but had no effect on cAMP-dependent secretion elicited either by VIP or forskolin. The results suggest that the antisecretory effect of vasopressin is mediated only by a block in the Ca2+ pathway, whereas SOM inhibits Ca2+-dependent secretion as well as receptor-mediated cAMP-dependent secretion. The interaction with the cAMP pathway is located at the step between stimulation of the receptor and activation of the adenylate cyclase and probably involves an Ni-protein.     

Functional vasoactive intestinal polypeptide (VIP)-system in salt glands of the Pekin duck

Summary In saltwater-acclimated ducks with fully specialized supraorbital salt glands, intracarotid application of acetylcholine (5 nmoles/min/kg b.w.) or porcine vasoactive intestinal polypeptide (pVIP) (240 pmoles/min/kg b.w.) induced secretion from the salt glands at threshold conditions of secretory activity. pVIP-like immunoreactivity could be localized in fibers of the postganglionic secretory nerve ramifying throughout the glandular parenchyma. Both middle-sized arterioles and secretory tubules were innervated, and pVIP-immunoreactive varicose fibers formed peritubular baskets around the basal region of secretory tubules indicating direct innervation of the secretory tissue. pVIP-specific staining could be abolished by preabsorption of the antiserum with peptide extracts of salt-gland tissue. Synthetic pVIP and endogenous VIP from salt glands of the duck co-eluted on the HPLC system, suggesting structural similarity of the peptides. Membrane-binding studies with radioiodinated pVIP revealed the presence of high-affinity binding sites in salt-gland tissue. Affinities of unlabeled pVIP analogues to compete for these binding sites were as follows: pVIP > PHI > pVIP antagonist > secretin > pVIP (10–28) > chicken VIP (16–28). Peptide extracts of salt glands had affinities similar to pVIP. Binding sites could be localized mainly at the apical end of the radially arranged secretory tubules, as demonstrated by receptor autoradiography.It is concluded that, in addition to the classical parasympathetic transmitter acetycholine, VIP serves as neuromodulator/transmitter in cranial parasympathetic control of avian salt-gland secretion by acting on both the arteriolar network and the secretory tubules of the gland.     

Effect of somatostatin infusion on VIP-induced transport changes in the human jejunum

This study was designed to elucidate the mechanism by which somatostatin administration ameliorates or abolishes diarrhea in pancreatic cholera syndrome (PCS). Absorption (or secretion) of water and electrolytes was measured in 30-cm segments of jejunum of 18 healthy volunteers in whom PCS was mimicked by intravenous infusion of VIP. Using the triple-lumen tube technique, the intestine was perfused with a plasma-like electrolyte solution while administering intravenous saline (control), VIP (400 pmol/kg/hr), somatostatin (5000 pmol/kg/hr), or VIP plus somatostatin. VIP infusion abolished water and electrolyte absorption and somatostatin had no effect on these VIP-induced transport changes regardless of whether somatostatin infusion was started before or after VIP infusion. Somatostatin infusion had no effect on VIP plasma concentration when elevated by intravenous VIP infusion (control: 10 +/- 1 pmol/l; during VIP infusion: 108 +/- 6). In a patient with pancreatic cholera syndrome identical perfusion experiments showed jejunal water secretion (93 ml/30 cm/hr) which changed to absorption (65 ml/30 cm/hr) when somatostatin was infused (5000 pmol/kg/hr). Plasma VIP concentration fell from 145 to 74 pmol/l (normal less than 50) during somatostatin infusion. Stool weight fell from 3722 g to 819 g per 24 hours when somatostatin was given at a dose of 2500 pmol/kg/hr for two days. Our observations in healthy subjects show that somatostatin has no effect on intestinal transport at the mucosal level when circulating VIP concentration is elevated.(ABSTRACT TRUNCATED AT 250 WORDS)     

Central mu, delta- and kappa-opioid influences on intestinal water and electrolyte transport in dogs

The effects of intracerebroventricular (i.c.v.) administration of opioid peptides with mu-(DAGO), mu- and delta-(DALAMIDE, DADLE) and kappa-(dynorphin) properties on normal and stimulated (cholera toxin) net fluxes of water, Na+ and K+ through a jejunal Thiry-Vella loop were investigated in conscious dogs. Basal net water absorption was slightly, but significantly (P less than 0.05) increased during i.c.v. infusion of DALAMIDE or DAGO (0.5 ng/kg/min) but not DADLE and dynorphin-(1-13) at the same rate; DALAMIDE and DAGO also markedly reduced (by 72.3 and 79.5% respectively) the secretory effects of cholera toxin (0.4 micrograms/ml). Similar effects were obtained with DALAMIDE and DAGO when injected i.c.v. as a bolus (100 ng/kg) prior to cholera toxin infusion; they were suppressed after i.v. pretreatment with naltrexone (0.3 mg/kg) but also with propranolol (0.2 mg/kg). In contrast, i.v. phentolamine (0.2 mg/kg) and bilateral truncal vagotomy, were unable to block their effects. These results suggest that Met-enkephalin can act centrally to affect intestinal transport of (i) water and (ii) electrolytes in dogs. They act probably at central mu-receptors which are involved in the regulation of intestinal secretion mediated through a central or peripheral beta-adrenergic pathway.     

Comparison of VIP-induced electrolyte secretion at three levels in rat small intestine.

Duodenal, jejunal and ileal loops were prepared and an iso-osmotic test solution injected, containing 80 mM Na+, 5-mM K+, 1.2 mM Ca2+, 77 mM Cl-, 10 mM HCO3- and 136 mM mannitol. 14CPEG 4000 was used as a non-absorbable marker and 36Cl was added to measure the bidirectional fluxes. During the 60-min in vivo incubation time, the duodenum actively secreted bicarbonate, a virtually zero flux in the jejunum was observed, whereas the ileum absorbed water and chloride and secreted bicarbonate. The response to the perfused doses of 0.15 to 2.4 nmol.100 g-1.h-1 of VIP (vasoactive intestinal peptide) differed qualitatively and quantitatively in the 3 segments: VIP increased bicarbonate secretion and induced chloride secretion in the duodenum, induced chloride secretion in the jejunum without changing bicarbonate minimal influx, induced bicarbonate secretion and suppressed chloride absorption in the ileum. The minimal dose required was lower in the duodenum (0.3 nmol.100 g-1.h-1) than in the jejunum and ileum (1.2 nmol.100 g-1.h-1). The functional heterogeneity of the small intestine was clearly demonstrated after VIP stimulation.     

Effects of angiotensin II, arginine-vasopressin, endothelin and catecholamines on plasma atrial natriuretic peptide concentrations in the conscious newborn calf.

The effects of epinephrine (E), norepinephrine (NE), angiotensin II (AII), arginine-vasopressin (AVP) and endothelin on plasma ANP levels were studied according to a latin square design in six 12-21 days-old conscious Jersey calves weighing 30 +/- 4 kg. The animals chronically-instrumented with a carotid catheter for blood pressure recording, received at 11.00 a.m. an i.v. right jugular continuous infusion for 30 min of two different sub-pressor or pressor dose-levels of each substance; E: 0.6 and 5.5 nmol/min per kg body wt; NE: 0.6 and 6 nmol/min per kg body wt; AII: 9.6 and 96 pmol/min per kg body wt; AVP: 0.6 and 69 pmol/min per kg body wt; and endothelin: 1.2 and 12 pmol/min per kg body wt). Control animals received, in the same way, the same volume (2 ml/kg body wt) of NaCl 0.9%. In Jersey calves, basal plasma atrial naturetic peptide (ANP) levels were around 5 pmol/l. Marked increases in this parameter were produced by all substances when given at the highest dose-level. The maximal rise of 600% was observed with AII; however on a molar basis, endothelin appeared more potent than AII and at the same dose-level, E appeared more effective than NE to increase circulating ANP (17.8 +/- 0.3 vs 9.5 +/- 0.1 respectively at time 70 min; P less than 0.01). The time-course of plasma ANP levels was positively correlated (P less than 0.01) by linear regression with the increase in blood pressure when pressor agents were given at the highest dose.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of vasoactive intestinal polypeptide on acetylcholine stimulation of rat submandibular gland

Studies were carried out on the role of vasoactive intestinal polypeptide (VIP) in the regulation of secretion and blood flow in the rat salivary gland. The first experiments to investigate the spontaneous secretory pattern revealed a clear diurnal fluctuation with a significant increase at night, so that the subsequent experiments were performed during the daytime where the secretion was consistently low. Intravenous administration of VIP at a dose smaller than 40 pmole caused a dose-dependent vasodilatory response, but at a high dose such a local effect was hampered by a decrease in systemic blood pressure. VIP potentiated the acetylcholine chloride (AcCho)--evoked salivary secretion, but VIP (0-100 pmole/kg) alone did not cause salivary secretion. Atropine reduced the salivary secretion evoked by AcCho and VIP, and the blood flow change evoked by AcCho. However, the blood flow change evoked by VIP was not affected by atropine. Hexamethonium exerted no significant effect on the response to administration of AcCho or VIP. The results indicate that VIP has a significant vasodilatory action and cooperates with AcCho in the regulation of salivary secretion in the rat, and VIP effects are atropine resistant, as in other species of animals.     

Vasoactive intestinal polypeptide: increased tone, enhancement of acetylcholine release, and stimulation of adenylate cyclase in intestinal smooth muscle

Vasoactive intestinal polypeptide (VIP) was examined $\text{in vitro}$ for effects on tone and neuronal release mechanisms in intestinal smooth muscle since this is a site of high peptide concentration. VIP contracted the guinea pig ileum and rabbit jejunum in concentrations ranging from 10^?9 to 10^?7 M. Increased tone in the guinea pig ileum was partially antagonized by the anticholinergic agent, atropine (4.38 × 10^?6 M) suggesting that one component of the contractile response was due to the indirect release of acetylcholine. The H₁ receptor antagonist, pyrilamine, did not alter the increased tone produced by VIP indicating that histamine release did not contribute to the ileal contractile response and that VIP exerted a selective effect to enhance neuronal release of acetylcholine. The ability of VIP to modulate acetylcholine release was confirmed in field stimulated ileal preparations where VIP increased the force developed to endogenously released acetylcholine without altering the direct response to acetylcholine. In rabbit jejunum and ileal smooth muscle, VIP related cyclic AMP levels. However, inhibition of phosphodiesterase with papaverine did not potentiate either the VIP-induced ileal contraction or enhancement of the field stimulated response. This raises the possibility that increases in intestinal cyclic AMP may be involved more in VIP-induced alterations in ion transport or secretory phenomenon than in intestinal motility. These studies describing the ability of VIP to modulate acetylcholine release and to increase ileal tone are consistent with the proposed role of VIP in intestinal patholgies involving excessive mucous secretion and motility.     

Comparison of helodermin, VIP and PHI in pancreatic secretion and blood flow in dogs

Helodermin, VIP and PHI, which share a high degree of homology with secretin, have been identified in the gut but their physiological role is unknown. In this study 3 series of tests were carried out to determine the actions of helodermin, VIP and PHI on pancreatic secretion in 6 conscious dogs and amylase release from the dispersed canine pancreatic acini and to correlate the alterations in pancreatic secretory and circulatory effects in 24 anesthetized dogs. Helodermin, VIP and PHI infused i.v. in graded doses (12.5-200 pmol/kg.h) resulted in a dose-dependent increase in pancreatic HCO3 secretion reaching, respectively, 100%, 7% and 2% of secretin maximum. When combined with constant dose infusion of CCK-8 (100 pmol/kg.h), helodermin but not VIP or PHI augmented dose-dependently the HCO3 secretion. When added in various concentrations (10(-10)-10(-5)M) to the incubation medium of dispersed pancreatic acini only helodermin but not VIP or PHI increased dose-dependently amylase release reaching about 50% of CCK-8 maximum. In anesthetized dogs, the pancreatic blood flow (PBF) measured by electromagnetic blood flowmetry showed an immediate and dose-dependent increase following the injections of various doses of helodermin, VIP, PHI and secretin, the peak blood flow preceding by about 1 min the peak secretory stimulation. This study shows that helodermin resembles secretin in its potent pancreatic HCO3 stimulation but differs from VIP or PHI which are poor secretagogues but potent vasodilators. We conclude that if tested peptides are released in the gut, helodermin, like secretin, may be involved in the hormonal stimulation of exocrine pancreas, whereas VIP and PHI may serve mainly as vasodilators in the pancreatic circulation.     

Effects of intraarterial, intravenous, and intraluminal neurotensin on canine ileal sodium and water absorption and blood flow

Neurotensin is a regulatory peptide which is found primarily in the ileum and is secreted into the blood and lumen. The physiologic effects of neurotensin are uncertain but in certain pathologic states neurotensin increases to levels which can have effects on many organs. The effects of intravenous, intraarterial and intraluminal neurotensin (0.075-7.5 micrograms/min) on fed canine ileal sodium and water fluxes, potassium secretion, and blood flows were studied. Intravenous and intraarterial infusion of neurotensin increased net sodium, potassium, and water secretion, due to increased secretory fluxes, and increased hematocrits. Intraarterial neurotensin was not more effective than intravenous neurotensin except for stimulating potassium secretion. Neurotensin increased potassium secretion at 0.075 micrograms/min IA, increased sodium and water secretion at 0.75 micrograms/min IA and IV, and increased hematocrit at 7.5 micrograms/min IA and and IV. Total and absorptive site blood flows and arterial and venous pressures were not changed. Intraluminal neurotensin had no effects at any infusion rate. Neurotensin can increase potassium secretion at physiologic levels by a local effect and can increase sodium and water secretion at high physiological-pathological levels through a hormonal mechanism. The secretion is not dependent on cardiovascular changes.     

Influence of electrical gradient on electrolyte and water transport in rabbit ileum and colon in vitro

We studied the influence of the electrical gradient between the mucous and serous side on water and electrolyte transport in the rabbit distal ileum and proximal colon "in vitro". In the absence of a gradient, the absorption of water, sodium, chloride and bicarbonate rose in both ileum and colon, whereas potassium secretion tended to give way to potassium absorption.     

CFTR基因在小鼠肠道组织中的差异表达

目的研究肠道组织CFTR基因表达与分泌性腹泻发生的关系。方法选取KM小鼠24只,雌雄各半,随机分为3组（每组8只）：对照组经小鼠腹腔注射0.2 mL生理盐水,实验组小鼠经腹腔注射LPS[6 mg/（kg·bw）]分别作用1 h、8 h,于注射后通过小鼠精神状态、肠道组织形态学判定分泌性腹泻模型的建立,利用荧光定量PCR法检测各段肠道组织CFTR基因的表达。结果 LPS成功诱导小鼠发生了分泌性腹泻;CFTR基因在小鼠十二指肠、空肠、回肠和结肠组织中均有不同的表达丰度,以结肠最高,但各段肠道间差异不显著;与对照组相比,LPS上调了十二指肠、空肠和回肠CFTR基因的转录,下调了结肠CFTR基因的转录。结论提示肠道组织CFTR基因转录水平的上调与LPS诱导分泌性腹泻的发生密切相关,且在各肠段发挥的作用不同,其中空肠在氯离子（Cl-）分泌中发挥主要作用,结肠的作用最弱。     

Effect of radiation on cAMP,cGMP and Ca(2+)(i) pathways and their interactions in rat distal colon

The secretory response implicated in the intestinal response to luminal attack is altered by radiation. The cAMP, cGMP and Ca(2+)(i) pathways leading to secretion as well as the interactions between the cAMP pathway and the cGMP or Ca(2+)(i) pathway were studied in the rat distal colon 4 days after a 9-Gy abdominal X irradiation, when modifications mainly occurred. The secretory response in Ussing chambers and cAMP and cGMP accumulation in single isolated crypts were measured. The muscarinic receptor characteristics were determined in mucosal membrane preparations. The secretory response by the cAMP pathway (stimulated by vasoactive intestinal peptide or forskolin) and the cAMP accumulation in crypts were decreased (P < 0.05) after irradiation. The weak secretory response induced by the cGMP pathway (stimulated by nitric oxide or guanylin) was unaltered by radiation, and the small amount of cGMP determined in isolated crypts from the control group became undetectable in the irradiated group. Inducible NOS was not involved in the hyporesponsiveness to VIP after irradiation (there was no effect of an iNOS inhibitor). The secretory response by the Ca(2+)(i) pathway (stimulated by carbachol) was unaffected despite a decreased number and increased affinity of muscarinic receptors. The non-additivity of VIP and carbachol co-stimulated responses was unmodified. In contrast, VIP and SNP co-stimulation showed that NO enhanced the radiation-induced hyporesponsiveness to VIP through a reduced accumulation of cAMP in crypts. This study provides further understanding of the effect of ionizing radiation on the intracellular signaling pathways.     

Evidence of luminal phosphatidylcholine secretion in rat ileum

BACKGROUND: Intestinal mucus not only facilitates substrate absorption, but also forms a hydrophobic, phosphatidylcholine (PC) enriched, barrier against luminal gut contents. METHODS: For evaluation of the origin of PC in intestinal mucus, we first analyzed the mucus PC in mice with absent biliary phospholipid secretion (mdr2 (-/-) mice) using electrospray ionization (ESI) tandem mass spectroscopy (MS/MS). Second, in situ perfused rat jejunum, ileum and colon were analyzed after i.v. bolus injections of 155 pmol [(3)H]-PC. Additional in vitro experiments were performed with isolated mucosal cells after incubation with the PC precursor [(3)H]-choline. RESULTS: In mdr2 (-/-) mice and control animals no significant quantitative difference in mucus PC was found, indicating that mucus PC is of intestinal and not biliary origin. In situ perfusion studies detected intestinal secretion of [(3)H]-PC, which was stimulated in presence of 2 mM taurocholate (TC). Secretion rates of [(3)H]-PC were highest in ileum (9.0+/-0.8 fmol h(-1)xcm(-1)), lower in jejunum (4.3+/-0.5) and minimal in colon (0.8+/-0.2). It compares to an intestinal secretion of native PC originating to 64% from bile, 9% from jejunum, 28% from ileum, and 1% from colon. Complementary in vitro studies showed 30-min secretion rates for [(3)H]-PC to be highest in enterocytes from ileum (26.5+/-5.3% of intracellular [(3)H]-PC) and jejunum (19.8+/-2.9%), and significantly lower in colonocytes (8.4+/-1.3%). CONCLUSION: PC in the intestinal mucus originates from secretion by ileal and jejunal enterocytes.     

Influence of VIP on the number of enterochromaffin and mucosal mast cells in the colon of the rat.

The possibility that VIP (Vasoactive intestinal peptide) could influence the enterochromaffin (EC) cell secretion of serotonin (5HT) and the action of VIP on the mast cell population of lamina propria were investigated in Wistar rat colon infused with a short chain fatty acid solution (sodium acetate), during a 1 h period. Under the action of an intravenous injection of synthetic porcine VIP, 14 micrograms/kg/h), the number of EC cells diminished significantly in the cecum and left colon, when compared to non-injected animals, both infused with a sodium acetate solution. At the same time, the number of mucosal mast cells in the crypts and lamina propria decreased significantly in the cecum. The postulate we put forward is that these VIP-induced changes are exerted through the stimulation of 5HT released from EC cells not only under normal physiological conditions but probably also under pathological conditions.     

A fast-acting humoral factor mediates pressor hyperresponsiveness in deoxycorticosterone-salt rabbits

Six rabbits were sham operated and were given water to drink (sham-water group); six additional rabbits were sham operated and were given saline to drink (sham-salt group); another six rabbits received an implant of deoxycorticosterone (DOCA) and were given water to drink (DOCA-water group); a final group of six rabbits received implants of DOCA and were given saline to drink (DOCA-salt group). Two weeks later, all four groups of rabbits had approximately the same mean arterial pressures, and the sham-salt, DOCA-water, and DOCA-salt groups all had plasma renin activity values less than the sham-water group. The DOCA-salt group had greater pressor responses to norepinephrine (NE) at several doses than did the other three groups of rabbits. In another group of six sham-water and six DOCA-salt rabbits, measurements of cardiac output before and during infusions of NE at 800 ng/min/kg body wt revealed no changes in cardiac output before or during NE infusion, but the DOCA-salt group had significantly greater increases in mean arterial pressure and total peripheral resistance during NE than did the sham-water group. In another group of six DOCA-salt rabbits, the pressor response to several doses of NE were determined during infusion of the angiotensin II (AII) antagonist, [Sar1, Ile8] AII; this AII antagonist failed to alter the enhanced pressor responses to NE. A final experiment examined pressor responses to NE in six normal rabbits before and after cross circulation of blood with six DOCA-salt rabbits. After blood cross circulation the normal rabbits had exaggerated pressor responses to NE at 5, 15, and 30 min, but not at 60 min. Similar cross-circulation experiments between six pairs of normal rabbits did not show any transfer of pressor hyperresponsiveness. These studies indicated that pressor and vascular hyperresponsiveness in DOCA-salt rabbits is conveyed by a fast-acting hormonal factor and that AII probably is not involved in mediating this hyperresponsiveness.     

Vasoactive intestinal polypeptide stimulates nonovarian progesterone secretion in rabbits

Recent experiments conducted in this laboratory have shown that intravenous infusions of vasoactive intestinal polypeptide (VIP) induced significant increases in plasma progesterone (P) in female rabbits. The purpose of this study was to determine the organ source of this P and to clarify the mechanisms by which it is induced. Intravenous infusions of VIP (37.5, 75, and 150 pmol/kg per min for 60 min) produced acute dose-dependent increases in plasma P in intact estrous rabbits. In ovariectomized (OVX) animals, VIP infusion (75 pmol/kg per min) produced a P increase of the same magnitude. In animals both OVX and adrenalectomized (ADX), this VIP effect was eliminated. The only significant change noted in luteotropic hormone (LH) or follicle stimulating hormone (FSH) was a decrease in FSH immediately following VIP infusion (150 pmol/kg). VIP infusion significantly increased plasma cortisol in intact and OVX animals, but not in OVX/ADX animals. It is concluded that VIP primarily stimulates the adrenal component of P secretion in the rabbit, via mechanisms independent of LH or FSH.     

Regulation mechanisms of intestinal secretion: implications in nutrient absorption

Intestinal secretion is a normal phenomenon, indispensible to solubilize and dilute nutrients and to maintain fluidity in the intestinal lumen. Enterotoxins and certain drugs may disrupt the proabsorptive status maintained by the small intestine under physiologic conditions. Hormones found in nervous and specialized intestinal enterochromaffin cells are responsible, in part, for secretion of fluid into the lumen. Afferent vagal nerve impulses mediated by 5-hydroxytryptamine (serotonin; 5-HT), vasoactive intestinal peptide (VIP) and substance P are the major agents of secretory stimulation. Toxins from pathogenic bacteria, especially some strains of E. coli and V. cholerae, trigger a secretory response and a chain of events involving cGMP and cAMP which result in chloride secretion, coupled to sodium and fluid efflux into the lumen. If secretion is unchecked by natural mechanisms or medications, the consequences are diarrhea, with potential dehydration, hyponatremia and ultimately death. Introduction of absorbable nutrients in the intestinal lumen has a major antisecretory action, both by a nutrient-gene interaction and by proabsorptive hormone expression. In additon, during the absorptive process water is carried into the enterocyte together with solutes. Hydrolysis-resistant peptides of dietary origin and ingested soluble fiber may also have a proabsorptive effect. The gastrointestinal system has a variety of antisecretory or proabsorptive hormonal and protein agonists that balance the outflow of fluid and electrolytes. The more extensively studied are neuropeptide Y/peptide YY (NPY/PYY) and the antisecretory factor (AF). Nitric oxide (NO), a short-lived second messenger, has a major role in secretion by activating cGMP. The intracellular concentration of NO may regulate the absorptive/secretory status of the small intestine, either stimulating absorption or inducing secretion. Specifically targeted 5-HT receptor antagonist drugs and other pharmacologic agents have been clinically tried for the treatment of severe diarrhea, drug-induced malabsorption and reversal of cellular damage.